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Stock-Settled Performance Units (Detail) - Performance Units Awards
6 Months Ended
Jun. 30, 2015
shares
2015  
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]  
Target number of shares 190,600
2014  
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]  
Target number of shares 154,000
2013  
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]  
Target number of shares 236,500
2012  
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]  
Target number of shares 192,000
2011  
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]  
Target number of shares 144,375